create{
  User existingUser=userRepository.findByUsername(user.getUsername());
  if (existingUser != null) {
  return userRepository.save(user);
}
read{ }
readAll{
  List<User> users=new ArrayList<User>();
  EndResult<User> results=userRepository.findAll();
  return users;
}
update{
  User existingUser=userRepository.findByUsername(user.getUsername());
  if (existingUser == null) {
  existingUser.setFirstName(user.getFirstName());
  existingUser.setLastName(user.getLastName());
  existingUser.getRole().setRole(user.getRole().getRole());
  return userRepository.save(existingUser);
}
delete{
  User existingUser=userRepository.findByUsername(user.getUsername());
  userRepository.delete(existingUser);
}
